The present invention relates to semiconductor materials, and, more particularly, to the passivation of mercury cadmium telluride (Hg.sub.1-x Cd.sub.x Te) and related materials by anodic selenidization.
Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation detection. For example, Hg.sub..8 Cd.sub..2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub..73 Cd.sub..27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors. In particular, p-n junction Hg.sub.1-x Cd.sub.x Te photodiode arrays have long been used (see, for example, Lorenze, U.S. Pat. No. 4,286,278), and extrinsic p type Hg.sub.1-x Cd.sub.x Te has potential application in infrared focal plane MIS detector arrays operating in the 10-12 .mu.m wavelength window. (Note that intrinsic p type Hg.sub.1-x Cd.sub.x Te, whose doping is presumably dominated by mercury vaccanies, was recently found to have midgap recombination centers proportional in concentration to the shallow acceptors; see C.Jones et al, 3 J. Vac.Sci.Tech. A 131 (1985). These recombination centers shorten minority carrier lifetimes and are sources of recombination-generation noise; thus extrinsic p type Hg.sub.1-x Cd.sub.x Te is preferred to intrinsic p type.) Such detectors are fabricated in large area Hg.sub.1-x Cd.sub.x Te that may be grown by LPE. MOCVD, MBE or bulk techniques and are operated typically at liquid nitrogen temperatures.
Passivation of Hg.sub.1-x Cd.sub.x Te prior to detector fabrication is necessary to avoid surface contamination by residues of various processing steps; such contamination affects the electrical characteristics of the detectors, for example, the photocarrier lifetime and stability. Analogous passivation of silicon for integrated circuits fabrication is typically achieved by growth of thermal oxides at temperatures about 1,000.degree. C.; however, thermal growth of oxides on Hg.sub.1-x Cd.sub.x Te is not feasible due to the out diffusion of mercury at even moderate temperatures. Consequently, passivation of Hg.sub.1-x Cd.sub.x Te by deposition of zinc sulfide or silicon dioxide has been used, but such passivation yields detectors that degrade (surface state density and accumulated surface charge vary and give unstable device characteristics) when subjected to temperatures over 70.degree. C. An improvement is passivation by anodic oxide: oxides of mercury, cadmium, and tellurium are grown on the surface of Hg.sub.1-x CD.sub.x Te electrochemically in a KOH solution; see Catagnus, U.S. Pat. No. 3,977,018, Anodic oxide is also temperature sensitive and yields detectors that degrade at about 80.degree. C. Further, even extended storage at room temperature degrades such detectors. Thus it is a problem to provide a passivation for Hg.sub.1-x Cd.sub.x Te that avoids detector degradation at temperatures somewhat above room temperature.